As used herein, "NO.sub.x " means the various compounds of nitrous oxide such as NO, NO.sub.2, N.sub.2 O etc. "Gas fired" or "gaseous fuel" means natural gas (including methane and small percentages of other elements commonly referred to as "street gas") and its higher order hydrocarbon derivatives such as butane, propane etc. "Turn down" will be used herein in the sense of air to fuel ratios and will refer to the ability of the burner to operate over a range of air to fuel ratios. "Combustion air" means the air we breathe in the atmosphere which contains some percentage of O.sub.2. This invention relates to a gas-fired burner producing little if any NO.sub.x emissions and specifically emissions low enough to meet currently contemplated standards such as those proposed in California which limit NO.sub.x emissions for industrial processes to 9 ppm.
In FIG. 2 of my '921 patent is a graph showing products of combustion and percentages thereof when a burner is operated at various air/fuel ratios including stoichiometric. In FIG. 3 of my '921 patent is a graph illustrating NO.sub.x compounds produced when an industrial burner is operated at various air/fuel ratios. In the text of the '921 patent, I discuss the beneficial effect of the presence of combustibles i.e. H.sub.2 and CO, produced by burners operating at low air/fuel ratios in preventing or retarding the formation of NO.sub.x compounds. It is also discussed that operating burners at low air/fuel ratios drops the adiabatic flame temperature of the burner to temperatures less than 2800.degree. F. and that at flame temperatures of less than 2800.degree. F., NO.sub.x formation will not occur whether or not combustibles such as H.sub.2 or CO are or are not present.
Finally, FIG. 5 of my '921 patent shows the adverse effects of preheated combustion air on NO.sub.x formation and discloses that air/fuel ratios in the range of 6 to 1 which would not produce NO.sub.x with cold air have to be lowered to the range of 5 to 1 if preheated combustion air is utilized. As is well known in the burner art, use of preheated combustion air, whether as secondary or primary combustion air, is desirable for fuel efficiencies.
My '883 patent discloses a burner which can operate at air/fuel ratios as low as 6.5 to 1 with cold air. In the '883 patent I tangentially inject into a cup shaped or cylindrical throat chamber a swirling mass of fuel/air mixture which is ignited therein. I sized the tangential injection opening to the throat chamber to be smaller than a chamber into which the air and fuel is admitted to create a back pressure which causes mixing of the air and fuel prior to being injected into the cup shaped throat chamber. The mixing of the air and fuel was sufficient to sustain ignition at air/fuel ratios which, at the time, were very low. However the mixing in the '883 device was not sufficient to prevent carbon formation or sooting at air/fuel ratios lower than 6.5 to 1 with cold air. Additionally, because of the lack of complete air fuel mixing prior to ignition, there was a tendency in the '883 burner to produce local adiabatic flame temperatures higher than 2800.degree. F. at air/fuel ratios lower than what otherwise would be possible because of further mixing of the air and fuel at the flame stabilization point in the burner.
With respect to prior art industrial burners operating at stoichiometric or excess air ratios, in my U.S. Pat. No. 4,214,866 there is disclosed a jet of combustion air actuating a swirler to cause mixing which progressively occurs downstream of the ignition point. With respect to a stoichiometrically operated burner which addresses the problems of carbon soot formation, reference can be had to Thekdi U.S. Pat. No. 3,951,584 which discloses a secondary air combustion zone which recirculates air back into a rich combustion zone to prevent carbon formation. Generally speaking, stoichiometric burners such as disclosed in the '866 and '584 patents, generate a fuel/air mixture which is capable of being ignited and then use various types of flow patterns, most often produced in combination with burner configuration, to cause thorough mixing of the fuel/air mixture as it longitudinally travels through or out of the burner. As noted, my '883 patent is distinguished from the other cited patents in that mixing of fuel/air occurred, at least partially, prior to ignition and this leads to the ability of the '883 patent to produce a burner which is capable of igniting low air/fuel ratios or ratios prior to this invention, which were considered low air/fuel ratios.